Deactivation of Microbubble Nucleation Sites by Alcohol-Water Exchange Xuehua Zhang,* ,†,‡ Henri Lhuissier, ‡ Oscar R. Enríquez, ‡ Chao Sun, ‡ and Detlef Lohse* ,‡ † Department of Chemical and Biomolecular Engineering and Particulate Fluid Processing Center, University of Melbourne, Parkville, VIC 3010, Australia ‡ Department of Science and Technology, J. M. Burgers Center for Fluid Dynamics and Mesa+, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands * S Supporting Information ABSTRACT: The ethanol-water exchange process is one of the standard methods of generating nanobubbles at a solid-water interface. In this work, we examine whether the nanobubbles formed by the solvent exchange can initiate microbubble formation as the temperature increases, thus acting as nuclei. This, however, is not the case: the nanobubbles are stable and do not facilitate microbubble formation. Instead, the process of solvent exchange, which aids the formation of nanobubbles and even microbubbles on some hydrophobic substrates under ambient conditions, suppresses microbubble nucleation on graphite and hydrophilic micropit-decorated substrates at high temperature (i.e., deactivates the nucleation sites for microbubble formation). We ascribe this behavior to the prewetting of the surface by the alcohol and the stability of the nanobubbles to the temperature increase. The findings in this study have implications for the prevention of bubble formation for a range of applications. ■ INTRODUCTION The “inverse” of water droplets on surfaces in air is air bubbles on surfaces in water. Just as droplets, they come in various sizes. What is currently intriguing in the field of colloid and surface science is surface nanobubbles (i.e., gaseous domains of nanoscale thickness). 1-3 These nanobubbles exist on hydro- phobic surfaces in contact with water and render the already complicated hydrophobic-water interfaces even more elusive. It has been reported that the presence of nanobubbles can influence a range of interfacial behaviors, such as thin liquid film rupture, 4 hydrodynamic boundary conditions, 5 particle or molecular adsorption, 6 and surface corrosion and catalysis processes. 7 Research efforts in the past decade have revealed that the formation of nanobubbles is closely related to the surface history. For example, a temperature gradient, electro- or photochemical reactions, or pressure fluctuations in the system can all induce nanobubbles. One of the most often used methods of producing nanobubbles is the solvent-exchange process 8 where a hydrophobic substrate is first exposed to a short-chain alcohol, such as ethanol or propanol, and then to water. Given that gases have a higher solubility in alcohol than in water, local gas supersaturation is created during the solvent exchange and nanobubbles form on the interface. 8 Although counterintuitive, once formed, the nanobubbles are very stable. Under ambient conditions, they can live for several days. 9 Several hypotheses have been proposed to explain the long life of surface nanobubbles, including a contamination shell on the bubble surface, 10 dynamic equilibrium theory, 11 and coexisting gas layers. 12,13 Recently, three research groups have independently proposed the important role of pinning at the three-phase boundary of nanobubbles and the gas saturation level in the liquid phase. 14-16 Nanobubbles also demonstrate strong stability under long, gentle sonication 17 and under a massive pressure reduction through a rarefaction wave. 18 In this Letter, we wonder how the formation process and the presence of surface nanobubbles influence microbubble nucleation with increasing temperature. Such a study may (i) shed light on the relation between nanobubbles and normal microbubbles from cavitation and (ii) provide a potential method of controlling microbubble nucleation in some practical processes. According to nucleation theory, the onset of bubble nucleation in a stationary system can be facilitated by micro/ nanoscale physical or chemical features on the substrate. 19-25 In particular, the presence of tiny gas pockets trapped inside the crevices on the surface, called Harvey nucleation sites, 24 has been used to rationalize the nucleation of bubbles, either under pressure reduction or under boiling conditions. Bremond et al. 26 have demonstrated that the bubbles specifically form on the built-in micropits on a flat background under pressure Received: May 25, 2013 Revised: July 26, 2013 Published: July 26, 2013 Letter pubs.acs.org/Langmuir © 2013 American Chemical Society 9979 dx.doi.org/10.1021/la402015q | Langmuir 2013, 29, 9979-9984